Magnetic multilayer pigment flake and coating composition

ABSTRACT

The present invention provides a magnetic multilayer pigment flake and a magnetic coating composition that are relatively safe for human health and the environment. The pigment flake includes one or more magnetic layers of a magnetic alloy and one or more dielectric layers of a dielectric material. The magnetic alloy is an iron-chromium alloy or an iron-chromium-aluminum alloy, having a substantially nickel-free composition. The coating composition includes a plurality of the pigment flakes disposed in a binder medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of commonly assigned and co-pendingU.S. patent application Ser. No. 15/812,824, filed Nov. 14, 2017, nowU.S. Pat. No. 10,479,901, issued on Nov. 19, 2019, which is aContinuation of commonly assigned and co-pending U.S. patent applicationSer. No. 15/296,863, filed Oct. 18, 2016, now U.S. Pat. No. 9,845,398,issued on Dec. 19, 2017, which is a Continuation of U.S. patentapplication Ser. No. 13/733,792, filed Jan. 3, 2013, now U.S. Pat. No.9,508,475, issued Nov. 29, 2016, which is a continuation-in-part of U.S.patent application Ser. No. 12/828,069 (abandoned) to Raksha et al.,filed on Jun. 30, 2010, the disclosures of which are hereby incorporatedby reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to multilayer pigment flakes and tocoating compositions incorporating such pigment flakes. In particular,the present invention relates to magnetic multilayer pigment flakes andto magnetic coating compositions.

BACKGROUND OF THE INVENTION

Chromium-containing materials are widely used in coating compositionsbecause of their advantageous optical-absorption andcorrosion-inhibiting properties. In many coating compositions, such asinterference coating compositions, layers of chromium-containingmaterials are used as absorbing layers in multilayer pigment flakes.

For example, as disclosed in U.S. Pat. No. 3,858,977 to Baird, et al.,issued on Jan. 7, 1975, in U.S. Pat. No. 5,059,245 to Phillips, et al.,issued on Oct. 22, 1991, in U.S. Pat. No. 5,571,624 to Phillips, et al.,issued on Nov. 5, 1996, in U.S. Pat. No. 6,132,504 to Kuntz, et al.,issued on Oct. 17, 2000, and in U.S. Pat. No. 6,156,115 to Pfaff, etal., issued on Dec. 5, 2000, which are incorporated herein by reference,layers of chromium metal may be used as absorbing layers. As disclosedin U.S. Pat. No. 4,978,394 to Ostertag, et al., issued on Dec. 18, 1990,and in U.S. Pat. No. 5,364,467 to Schmid, et al., issued on Nov. 15,1994, which are incorporated herein by reference, layers ofchromium(III) oxide (Cr₂O₃) may be used as absorbing layers. Asdisclosed in U.S. Pat. No. 5,424,119 to Phillips, et al., issued on Jun.13, 1995, in U.S. Pat. No. 6,235,105 to Hubbard, et al., issued on May22, 2001, in U.S. Pat. No. 6,524,381 to Phillips, et al., issued on Feb.25, 2003, in U.S. Pat. No. 6,648,957 to Andes, et al., issued on Nov.18, 2003, in U.S. Pat. No. 6,759,097 to Phillips, et al., issued on Jul.6, 2004, in U.S. Pat. No. 6,818,299 to Phillips, et al., issued on Nov.16, 2004, and in U.S. Pat. No. 7,169,472 to Raksha, et al., issued onJan. 30, 2007, which are incorporated herein by reference, layers ofcommonly available chromium-containing alloys, such as Hastelloys,Inconels, stainless steels, and nickel-chromium alloys, may be used asabsorbing layers.

Unfortunately, many of the chromium-containing materials in theabsorbing layers of prior-art coating compositions are harmful to humanhealth. Chromium metal and chromium(III) oxide, for example, each causeirritation to the skin, eyes, respiratory tract, and gastrointestinaltract. Moreover, these materials may be oxidized to form chromium(VI)species, which are, generally, toxic and carcinogenic. Furthermore, thechromium-containing alloys used in the absorbing layers of prior-artcoating compositions, typically, also contain nickel, which is toxic andcarcinogenic. Therefore, many prior-art coating compositions based onchromium-containing materials pose potential health and environmentalhazards.

Despite their advantageous corrosion-inhibiting properties, inparticular, chromium-containing magnetic alloys are not, generally, usedas magnetic layers in multilayer magnetic pigment flakes. Rather, asdisclosed in U.S. Pat. No. 6,808,806 to Phillips, et al., issued on Oct.26, 2004, in U.S. Pat. No. 6,818,299, and in U.S. Pat. No. 7,169,472,additional dielectric or insulator layers are, conventionally, used toimprove the corrosion resistance of magnetic layers.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the shortcomings ofthe prior art by providing a magnetic multilayer pigment flake and amagnetic coating composition that are relatively safe for human healthand the environment.

Accordingly, the present invention relates to a magnetic multilayerpigment flake comprising: one or more magnetic layers of a magneticalloy, wherein the magnetic alloy is an iron-chromium alloy or aniron-chromium-aluminum alloy, having a substantially nickel-freecomposition; and one or more dielectric layers.

Another aspect of the present invention relates to a magnetic coatingcomposition comprising: a binder medium; and a plurality of magneticmultilayer pigment flakes disposed in the binder medium, wherein theplurality of pigment flakes each comprise: one or more magnetic layersof a magnetic alloy, wherein the magnetic alloy is an iron-chromiumalloy or an iron-chromium-aluminum alloy, having a substantiallynickel-free composition; and one or more dielectric layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in greater detail with referenceto the accompanying drawings, which represent exemplary embodimentsthereof, wherein:

FIG. 1A is a schematic illustration of a cross-section of a firstpreferred embodiment of a magnetic multilayer pigment flake;

FIG. 1B is a plot of the angle-dependent color travel of a magneticcoating composition comprising a plurality of the pigment flakes of FIG.1A having the following layer structure: Fe—Cr, semi-transparent/MgF₂,370 nm/Al, opaque/MgF₂, 370 nm/Fe—Cr, semi-transparent;

FIG. 1C is a plot of the angle-dependent color travel of a magneticcoating composition comprising a plurality of the pigment flakes of FIG.1A having the following layer structure: Fe—Cr—Al,semi-transparent/MgF₂, 370 nm/Fe—Cr—Al, opaque/MgF₂, 370 nm/Fe—Cr—Al,semi-transparent;

FIG. 2A is a schematic illustration of a cross-section of a secondpreferred embodiment of a magnetic multilayer pigment flake having afirst layer-thickness profile;

FIG. 2B is a schematic illustration of a cross-section of a secondpreferred embodiment of a magnetic multilayer pigment flake having asecond layer-thickness profile;

FIG. 3A is a schematic illustration of a preferred embodiment of acoating composition being exposed to microwave radiation; and

FIG. 3B is a schematic illustration of the coating compositionillustrated in FIG. 3A being exposed to a magnetic field.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a magnetic multilayer pigment flake and amagnetic coating composition incorporating such pigment flakes. Thepigment flake and, consequently, the coating composition substantiallypreclude the release of potentially harmful nickel and chromium(VI),while providing advantageous magnetic, optical, and corrosion-inhibitingproperties.

The pigment flake includes a plurality of thin-film layers of variousmaterials. Generally, the pigment flake has an aspect ratio of at least2:1 and an average particle size of about 2 μm to about 20 μm.

In particular, the pigment flake includes one or more magnetic layers ofa magnetic alloy, i.e. a ferro- or ferrimagnetic alloy, enabling thepigment flake to be aligned with a magnetic field. The magnetic alloyhas a nickel-free composition including iron and chromium. Optionally,the composition of the magnetic alloy may also include other metals,such as aluminum, minor constituents, and/or impurities, e.g. carbonand/or silicon on the scale of about 0.1 wt %. Preferably, thecomposition of the magnetic alloy consists essentially of iron andchromium or of iron, chromium, and aluminum. For example, the magneticalloy may be an iron-chromium alloy or an iron-chromium-aluminum alloy.

In the magnetic alloy, the chromium atoms are bonded by metallic bonds,which involve the sharing of electrons. Thus, chromium is present in themagnetic alloy as chromium(0). If the magnetic alloy is subject tocorrosion, chromium is mainly released as chromium(III), rather thanpotentially harmful chromium(VI). Moreover, a chromium(III)-containingoxide may be formed, which passivates the surface of the magnetic alloy,inhibiting further corrosion.

The inventors have found that a composition of the magnetic alloyincluding about 5 wt % to about 30 wt % chromium, about 0 wt % to about18 wt % aluminum, and a balance of iron minimizes the undesirablerelease of chromium(VI), but retains desirable magnetic, optical, andcorrosion-inhibiting properties. Preferably, the pigment flake releasessubstantially no chromium(VI). Moreover, the inclusion of aluminum inthe magnetic alloy provides increased reflectance.

Preferably, the composition of the magnetic alloy consists essentiallyof about 5 wt % to about 30 wt % chromium, about 0 wt % to about 18 wt %aluminum, and a balance of iron. That is, the magnetic alloy may be aniron-chromium alloy having a composition consisting essentially of about5 wt % to about 30 wt % chromium, and a balance of iron, or may be aniron-chromium-aluminum alloy having a composition consisting essentiallyof about 5 wt % to about 30 wt % chromium, greater than 0 wt % to about18 wt % aluminum, and a balance of iron.

In a preferred embodiment, which provides advantageousoptical-absorption properties, the magnetic alloy is an iron-chromiumalloy having a composition consisting essentially of about 5 wt % toabout 15 wt % chromium and a balance of iron. In some instances, themagnetic iron-chromium alloy has a composition consisting essentially ofabout 10 wt % to about 12 wt % chromium and a balance of iron.

In another preferred embodiment, which provides advantageousoptical-reflection properties, the magnetic alloy is aniron-chromium-aluminum alloy having a composition consisting essentiallyof about 10 wt % to about 30 wt % chromium, about 1 wt % to about 18 wt% aluminum, and a balance of iron. In some instances, the magneticiron-chromium-aluminum alloy has a composition consisting essentially ofabout 18 wt % to about 25 wt % chromium, about 6 wt % to about 15 wt %aluminum, and a balance of iron.

It should be noted that the composition of the magnetic alloy maydiffer, sometimes considerably, in different pigment flakes within thesame batch of pigment flakes due to local variations in the depositedcomposition.

The pigment flake, typically, includes a plurality of magnetic layers ofthe magnetic alloy, in addition to a plurality of dielectric layers.Optionally, the pigment flake may also include layers of other types.

The magnetic layers of the magnetic alloy, typically, serve as absorbinglayers for absorbing light and/or as reflecting layers for reflectinglight. In other words, the pigment flake may include one or moreabsorbing magnetic layers and/or one or more reflecting magnetic layers.The magnetic layers may be formed of the same or different magneticalloys and may have the same or different physical thicknesses. Forexample, the pigment flake may include one or more absorbing magneticlayers and one or more reflecting magnetic layers formed of the samemagnetic alloy, but having different physical thicknesses. Generally,the magnetic layers each have a physical thickness of about 3 nm toabout 1000 nm. In instances where the magnetic layers serve as absorbinglayers, the magnetic layers are semi-transparent, each, typically,having a physical thickness of about 3 nm to about 50 nm. Preferably,such semi-transparent absorbing magnetic layers each have a physicalthickness of about 5 nm to about 15 nm. In instances where the magneticlayers serve as reflecting layers, the magnetic layers are opaque, each,typically, having a physical thickness of about 20 nm to about 1000 nm.Preferably, such opaque reflecting magnetic layers each have a physicalthickness of about 50 nm to about 100 nm. Generally, the magnetic layersare amorphous, having been deposited by evaporation in vacuum.

In some instances, an opaque layer of a reflective material other thanthe magnetic alloy may serve as a reflecting layer for reflecting light.Suitable reflective materials include tin, aluminum, copper, silver,gold, palladium, platinum, titanium, and compounds or alloys thereof.Such an opaque reflecting layer is, preferably, formed of aluminum.Typically, such an opaque reflecting layer has a physical thicknesswithin the same ranges as the opaque reflecting magnetic layers.

The dielectric layers, typically, serve as transparent spacer layers,and provide the pigment flake with durability and rigidity. Thedielectric layers may be formed of any transparent dielectric materialhaving a low refractive index, i.e. a refractive index of less thanabout 1.65, or a high refractive index, i.e. a refractive index ofgreater than about 1.65. Suitable dielectric materials having a lowrefractive index include silicon dioxide (SiO₂), aluminum oxide (Al₂O₃),and metal fluorides, such as magnesium fluoride (MgF₂). Suitabledielectric materials having a high refractive index include siliconmonoxide (SiO) and zinc sulfide (ZnS). Preferably, the dielectric layersare formed of magnesium fluoride.

The dielectric layers may be formed of the same or different dielectricmaterials and may have the same or different physical thicknesses. Forexample, the pigment flake may include one or more dielectric layersformed of the same dielectric material, but having different physicalthicknesses and, therefore, different optical thicknesses. Generally,the dielectric layers each have a physical thickness of about 100 nm toabout 5000 nm. The physical thickness is selected to correspond with anoptical thickness required by a layer structure of the pigment flake forproviding a desired optical effect.

The pigment flake may have a variety of layer structures, having variouscompositional and layer-thickness profiles, for providing a variety ofoptical effects. Typically, the pigment flake has an interference layerstructure. Preferably, the pigment flakes has an interference layerstructure for providing a color-shifting effect through the interferenceof light, such that the pigment flake changes color with viewing angleor angle of incident light.

With reference to FIG. 1A, a first preferred embodiment of the pigmentflake 100 has a symmetrical interference layer structure including fivelayers: two semi-transparent absorbing magnetic layers 110 and 112, twotransparent dielectric layers 120 and 121, and one opaque reflectinglayer 111, which may be non-magnetic or magnetic. A first transparentdielectric layer 120 overlies a first semi-transparent absorbingmagnetic layer 110, a central opaque reflecting layer 111 overlies thefirst transparent dielectric layer 120, a second transparent dielectriclayer 121 overlies the central opaque reflecting layer 111, and a secondsemi-transparent absorbing magnetic layer 112 overlies the secondtransparent dielectric layer 121.

The first and second semi-transparent absorbing magnetic layers 110 and112 are formed of the magnetic alloy, and the first and secondtransparent dielectric layers 120 and 121 are formed of a dielectricmaterial, as described heretofore.

In some embodiments, the central opaque reflecting layer 111 is formedof a reflective material other than the magnetic alloy, as describedheretofore. To illustrate such an embodiment, a layer stack wasfabricated having the following layer structure: Fe—Cr,semi-transparent/MgF₂, 370 nm/Al, opaque/MgF₂, 370 nm/Fe—Cr,semi-transparent. First and second semi-transparent absorbing magneticlayers 110 and 112 of an iron-chromium alloy, first and secondtransparent dielectric layers 120 and 121 of magnesium fluoride, and acentral opaque reflecting layer 111 of aluminum were deposited byevaporation in vacuum onto a polyester substrate. The iron-chromiumalloy had a composition consisting essentially of about 10 wt % to about12 wt % chromium and a balance of iron.

The layer stack was stripped from the substrate and ground to form aplurality of pigment flakes 100 having an average particle size of about20 μm. The plurality of pigment flakes 100 were combined with a bindermedium to form a coating composition, and the coating composition wasprinted onto a paper substrate and dried. The color-shifting propertiesof the printed coating composition were then analyzed with agoniospectrophotometer. The angle-dependent color travel of the printedcoating composition with a change of viewing angle from 10° to 60° isplotted in FIG. 1B.

In other embodiments, the central opaque reflecting layer 111 is formedof the magnetic alloy, preferably, embodied as an iron-chromium-aluminumalloy, such that the magnetic layers 110, 111, and 112 alternate withthe dielectric layers 120 and 121. To illustrate such an embodiment, alayer stack was fabricated having the following layer structure:Fe—Cr—Al, semi-transparent/MgF₂, 370 nm/Fe—Cr—Al, opaque/MgF₂, 370nm/Fe—Cr—Al, semi-transparent. First and second semi-transparentabsorbing magnetic layers 110 and 112 of an iron-chromium-aluminumalloy, first and second transparent dielectric layers 120 and 121 ofmagnesium fluoride, and a central opaque reflecting magnetic layer 111of the iron-chromium-aluminum alloy were deposited by evaporation invacuum onto a polyester substrate to form the layer stack. Theiron-chromium-aluminum alloy had a composition consisting essentially ofabout 18 wt % to about 25 wt % chromium, about 6 wt % to about 15 wt %aluminum, and a balance of iron.

The layer stack was stripped from the substrate and ground to form aplurality of pigment flakes 100 having an average particle size of about20 μm. The plurality of pigment flakes 100 were combined with a bindermedium to form a coating composition, and the coating composition wasprinted onto a paper substrate and dried. The color-shifting propertiesof the printed coating composition were then analyzed with agoniospectrophotometer. The angle-dependent color travel of the printedcoating composition with a change of viewing angle from 10° to 60° isplotted in FIG. 1C.

Advantageously, embodiments of the pigment flake that include aplurality of magnetic layers of the magnetic alloy in alternation with aplurality of dielectric layers absorb microwave radiation particularlywell, allowing the pigment flake to be heated with microwave radiation.In such embodiments, the magnetic alloy serves three differentfunctions: enabling microwave absorption by the pigment flake, enablingoptical absorption by the pigment flake, and enabling magnetic alignmentof the pigment flake.

In some embodiments, the pigment flake includes inner and outer groupsof dielectric layers having different optical thicknesses and differentfunctionalities. Typically, the dielectric layers of the inner grouphave one or more optical thicknesses selected to provide resonantmicrowave absorption, whereas the dielectric layers of the outer grouphave one or more optical thicknesses, different from those of the innergroup, selected to provide an interference color.

With reference to FIGS. 2A and 2B, a second preferred embodiment of thepigment flake 200 a/200 b has a symmetrical interference structureincluding eleven layers: two semi-transparent absorbing magnetic layers210 a/210 b and 215 a/215 b, five transparent dielectric layers 220a/220 b, 221 a/221 b. 222 a/222 b. 223 a/223 b, and 224 a/224 b, andfour opaque reflecting magnetic layers 211 a/211 b, 212 a/212 b, 213a/213 b, and 214 a/214 b. In the illustrated embodiment, the magneticlayers 210 a/210 b, 211 a/211 b. 212 a/212 b. 213 a/213 b, 214 a/214 b,and 215 a/215 b are all formed of the same magnetic alloy. Likewise, thedielectric layers 220 a/220 b, 221 a/221 b. 222 a/222 b. 223 a/223 b,and 224 a/224 b are all formed of the same dielectric material.

A first transparent dielectric layer 220 a/220 b overlies a firstsemi-transparent magnetic layer 210 a/210 b, a first opaque reflectingmagnetic layer 211 a/211 b overlies the first transparent dielectriclayer 220 a/220 b, a second transparent dielectric layer 221 a/221 boverlies the first opaque reflecting magnetic layer 211 a/211 b, asecond opaque reflecting magnetic layer 212 a/212 b overlies the secondtransparent dielectric layer 221 a/221 b, a third transparent dielectriclayer 222 a/222 b overlies the second opaque reflecting magnetic layer212 a/212 b, a third opaque reflecting magnetic layer 213 a/213 boverlies the third transparent dielectric layer 222 a/222 b, a fourthtransparent dielectric layer 223 a/223 b overlies the third opaquereflecting magnetic layer 213 a/213 b, a fourth opaque reflectingmagnetic layer 214 a/214 b overlies the fourth transparent dielectriclayer 223 a/223 b, a fifth transparent dielectric layer 224 a/224 boverlies the fourth opaque reflecting magnetic layer 214 a/214 b, and asecond semi-transparent absorbing magnetic layer 215 a/215 b overliesthe fifth transparent dielectric layer 224 a/224 b, such that themagnetic layers 210 a/210 b, 211 a/211 b, 212 a/212 b, 213 a/213 b, 214a/214 b, and 215 a/215 b alternate with the dielectric layers 220 a/220b, 221 a/221 b, 222 a/222 b, 223 a/223 b, and 224 a/224 b.

The first and second semi-transparent absorbing magnetic layers 210a/210 b and 215 a/215 b, and the first, second, third, and fourth opaquereflecting magnetic layers 211 a/211 b, 212 a/212 b, 213 a/213 b, and214 a/214 b are formed of the magnetic alloy. The first, second, third,fourth, and fifth transparent dielectric layers 220 a/220 b, 221 a/221b, 222 a/222 b, 223 a/223 b, and 224 a/224 b are formed of a dielectricmaterial, as described heretofore.

The pigment flake 200 a/200 b may have various layer-thickness profilesselected to optimize resonant microwave absorption over a largebandwidth. With particular reference to FIG. 2A, according to a firstlayer-thickness profile of the pigment flake 200 a, the first, second,third, and fourth opaque reflecting magnetic layers 211 a, 212 a, 213 a,and 214 a have the same physical thickness, which is larger than that ofthe first and second semi-transparent absorbing magnetic layers 210 aand 215 a. The second, third, and fourth transparent dielectric layers221 a, 222 a, and 223 a have the same physical thickness, which issmaller than that of the first and fifth transparent dielectric layers220 a and 224 a.

With particular reference to FIG. 2B, according to a secondlayer-thickness profile of the pigment flake 200 b, the physicalthickness of the second and third opaque reflecting magnetic layers 212b and 213 b is larger than that of the first and fourth opaquereflecting magnetic layers 211 b and 214 b, which is larger than that ofthe first and second semi-transparent absorbing magnetic layers 210 band 215 b. The physical thickness of the third transparent dielectriclayer 222 b is smaller than that of the first and fifth transparentdielectric layers 220 b and 224 b, which is smaller than that of thesecond and fourth transparent dielectric layers 221 b and 223 b.Advantageously, such a layer-thickness profile provides a particularlylarge bandwidth of microwave absorption.

Of course, numerous other embodiments of the pigment flake provided bythe present invention may be envisaged without departing from the spiritand scope of the invention.

The pigment flake of the present invention can be formed by variousfabrication methods, as disclosed in U.S. Pat. No. 5,059,245, in U.S.Pat. No. 5,571,624, in U.S. Pat. No. 6,524,381, and in U.S. Pat. No.6,818,299, for example. Generally, some or all of the component layersare sequentially deposited on a substrate by using a conventionaldeposition technique, such as a physical vapor deposition (PVD),chemical vapor deposition (CVD), or electrolytic deposition, to form alayer stack.

For example, the magnetic layers may be deposited by evaporating a wireof a magnetic alloy, e.g. a ferritic stainless steel or Kanthal alloy,in vacuum. It should be noted that the composition of the magnetic alloyin the deposited magnetic layers often differs from that of the wire.Moreover, the composition of the magnetic alloy may differ, sometimesconsiderably, at different points in the deposited magnetic layer.

The layer stack is subsequently stripped from the substrate and groundto form a plurality of pigment flakes or preflakes. If preflakes areformed, the remaining component layers are then sequentially depositedon the preflakes to form a plurality of pigment flakes.

The plurality of pigment flakes may be combined with a binder medium toproduce the coating composition of the present invention. Typically, thebinder medium includes a resin that can be cured, for example, byevaporation, by heating, or by exposure to ultraviolet (UV) radiation.Suitable resins include alkyd resins, polyester resins, acrylic resins,polyurethane resins, vinyl resins, epoxy resins, styrene resins, andmelamine resins. Optionally, the binder medium may include a solvent,such as an organic solvent or water, a cure retarder, such as clove oil,or other additives.

The coating composition may be used as a paint or an ink and applied tovarious objects, such as currency and security documents, productpackagings, fabrics, motorized vehicles, sporting goods, electronichousings, household appliances, architectural structures, and floorings.Preferably, the coating composition is an interference coatingcomposition providing a color-shifting effect through the interferenceof light.

Being relatively safe for human health and the environment, the coatingcomposition is well-suited for use in applications where chemical safetyis a concern and for use under conditions where chemical release islikely to occur.

Being magnetic, the coating composition is also well-suited for use inprinting optical-effect images, such as three-dimensional, illusionary,and/or kinematic images, by aligning the magnetic pigment flakes withinthe coating composition with a magnetic field. A variety ofoptical-effect images for decorative and security applications can beproduced by various methods, as disclosed in U.S. Pat. No. 6,759,097, inU.S. Pat. No. 7,047,883 to Raksha, et al., issued on May 23, 2006, inU.S. Patent Application Publication No. 2006/0081151 to Raksha, et al.,published on Apr. 20, 2006, and in U.S. Patent Application PublicationNo. 2007/0268349 to Kurman, published on Nov. 22, 2007, for example,which are incorporated herein by reference.

Generally, the coating composition is printed on a substrate by aconventional printing technique, such as gravure, stamping, intaglio,flexographic, silk-screen, jet, or lithographic printing. While stillfluid or after being re-fluidized, the coating composition is exposed toa magnetic field, which aligns the magnetic pigment flakes within thecoating in a desired pattern. The binder medium within the coatingcomposition is then cured, for example, by evaporation, by heating, orby exposure to UV radiation, fixing the alignment of the pigment flakesin the desired pattern to form the optical-effect image.

With reference to FIGS. 3A and 3B, a preferred embodiment of the coatingcomposition 330, which is well-suited for use as an intaglio ink,includes pigment flakes 300 that absorb microwave radiation 340 disposedin a high-viscosity binder medium 350. The coating composition 330 isprinted on a substrate 360. With particular reference to FIG. 3A, whenthe coating composition 330 is exposed to microwave radiation 340, thepigment flakes 300 absorb the microwave radiation 340, generating heat.The generated heat reduces the viscosity of the binder medium 350 inmicrocapsules 351 surrounding the pigment flakes 300. Advantageously, itis only necessary to apply enough microwave radiation 340 to the coatingcomposition 330 to reduce the viscosity within the microcapsules 351,rather than within the binder medium 350 as a whole.

With particular reference to FIG. 3B, when the coating composition 330is soon afterward exposed to a magnetic field 370, the pigment flakes300, which are free to move within the low-viscosity microcapsules 351,align themselves with the magnetic field 370. The coating composition330 is then removed from the magnetic field 370 and is cured byevaporation, fixing the alignment of the pigment flakes 300. Althoughthe pigment flakes 300 are illustrated in FIG. 3B as being alignedparallel to the substrate 360, the pigment flakes 300 may be aligned innumerous other patterns by varying the direction and intensity of themagnetic field 370.

Of course, numerous other embodiments of the coating compositionprovided by the present invention may be envisaged without departingfrom the spirit and scope of the invention.

We claim:
 1. A method for making a multilayer pigment flake comprising:forming on a substrate a layer stack including two or moresemi-transparent absorber layers of a ferromagnetic alloy, two or moredielectric layers, and one or more opaque reflecting layers of theferromagnetic alloy; stripping the layer stack from the substrate; andgrinding the layer stack to form a plurality of multilayer pigmentflakes; and wherein the ferromagnetic alloy is a nickel-freecomposition, and consists essentially of about 5 wt. % to about 30 wt. %chromium, about 0 wt. % to about 18 wt. % aluminum, and a balance ofiron.
 2. The method of claim 1, further comprising: utilizing theplurality of multilayer pigment flakes as preflakes; and sequentiallydepositing additional component layers on the preflakes to form aplurality of pigment flakes.
 3. The method of claim 1, wherein the layerstack was formed using evaporation in vacuum.
 4. The method of claim 1,wherein the substrate is a polyester substrate.
 5. The method of claim1, wherein the two or more dielectric layers have different opticalthicknesses.
 6. The method of claim 1, wherein the layer stack is formedby a deposition technique chosen from physical vapor deposition,chemical vapor deposition, or electrolytic deposition.
 7. The method ofclaim 1, wherein the multilayer pigment flakes have an average particlesize of about 20 μm.
 8. The method of claim 7, wherein the multilayerpigment flakes are combined with a hinder medium to form a coatingcomposition.
 9. The method of claim 8, further comprising printing thecoating composition onto at least one of currency, security documents,and product packagings.
 10. The method of claim 8, further comprisingcuring the binder medium by a process chosen from evaporation, heating,or exposure to ultraviolet radiation.
 11. The method of claim 8, whereinthe binder medium includes an additive chosen from an organic solvent,water, or a cure retarder.
 12. The method of claim 8, further comprisingprinting the coating composition onto a paper substrate.
 13. The methodof claim 12, further comprising heating the coating composition withmicrowave radiation.
 14. The method of claim 13, further comprisingexposing the coating composition to a magnetic field to align themultilayer pigment flakes.
 15. The method of claim 14, furthercomprising forming an optical-effect image.
 16. The method of claim 14,wherein the aligned multilayer pigment flakes exhibit angle-dependentcolor travel.